Effect of fine particle bombarding on fatigue strength of high strength steel

The cantilever-type rotary bending fatigue test was performed at room temperature on high carbon chromium steel. Three kinds of specimens were prepared for comparison, i.e. fine particle bombarded, shot peened and untreated. Each specimen showed a step-wise S-N curve which had a transition stress, transferring the fatigue fracture initiation from surface to subsurface with decreasing the applied stress. The effect of the surface treatment appeared at stress levels above the transition stress of untreated specimen, while it disappeared at stress levels below that stress. The subsurface fracture seemed to initiate from the inclusion in the place where the superposition stress, which was the sum of the applied stress and the residual stress, was the largest. Although the features of fish-eyes were somewhat different in three kinds of specimens, the fatigue lives were almost the same. Regardless of the surface treatment, the fatigue life increased as the initial stress intensity factor, obtained by assuming the inclusion of a fracture origin as a crack, decreased. It is considered in the case of subsurface fracture that the fatigue strength is not influenced remarkably by the surface treatment.